Method for measuring tritium or other radiation for dismantling

ABSTRACT

The invention relates to a kit for detecting β-radiation on a solid surface in a manner that is not destructive to the solid surface, said kit including at least two films that are sensitive to at least two different types of radiation, including the β-type.

FIELD OF THE INVENTION

The invention relates to the field of radioactive radiations detection.More particularly, the invention relates to β⁻ radiation detection onsolid surfaces. Pure β⁻ emitters are radionuclides the β⁻ radiation ofwhich is not accompanied by X or γ radiation.

TECHNOLOGICAL BACKGROUND

It is important to be able to detect radioactive radiations on a nuclearsite, in particular before dismantling thereof, in order to know therisks that the persons present on the site are incurring and to providethem with suitable protection.

Certain radiations are easier to detect than others. For theseradiations, such as the γ (gamma) and X types, suitable tools areavailable today for knowing the intensity of these radiations in alocalised manner.

However, other radiations are more difficult to detect since these aretoo weak, such as those of the β⁻ type. β⁻ radiations are emitted byatom nuclei, comprising neutrons and protons, presenting an excess ofneutrons. At constant pressure, a neutron of the nucleus may transforminto a proton. This is accompanied by the emission of an electron, whichis a β⁻ particle, and an antineutrino. β⁻ radiations are termedmoderately penetrating; a sheet of aluminium a few millimetres thicksuffices to stop β⁻ radiations, unlike γ and X radiations, which aretermed highly penetrating.

In nuclear sites, β⁻ radiations are due mainly to tritium ³H, aradioactive isotope of hydrogen ¹H. Before proceeding with thedismantling of the site, it is wished to identify the areas emitting β⁻radiations, which reveals the presence of tritium ³H that it isnecessary to decontaminate.

At the present time, in order to detect β⁻ radiations due to tritium ³Hon a solid surface, samplings are carried out, which destroys the solidsurface being studied.

For example, in the document “Calculations and measurements of theactivation induced in the protective concretes of a high-energy ionaccelerator”, in Radioprotection 2000, volume 35, n° 3, pages 311 to334, the dismantling of the Saturn II synchrotron is described, inoperation from 1979 to 1997 at Saclay, France. Prior to the measurement,core sampling is carried out in the concrete of the synchrotronstructure, and then the core taken is sampled with diamond. The tritium³H is then measured on these cores by counting with a liquidscintillator. The sample preparation method is not known from thisdocument.

One drawback of the method used in this article is the need to destroythe solid surface on which it is wished to carry out the measurements oftritium ³H. Another drawback is the very localised measurement oftritium ³H and, if a measurement at another place is required, anothercore sampling is necessary.

A device and method for measuring tritium ³H in a non-destructive mannerare described in the document JP 3-041 386. The device comprises aconnection head composed of one end of an aspiration tube, heatingelements and fixing suckers for fixing to a solid surface. Theaspiration tube is connected to a cold trap, itself connected to anaspiration pump provided with a flow meter.

The method used is as follows: the connection head is placed on thesolid surface, in this case concrete. The fixing suckers hold theconnection head in place. The heating elements heat the concrete 200° C.on the part covered by the connection head, thus evaporating the waterpresent in the concrete. The pump sucks the water vapour escaping fromthe concrete. When the water vapour reaches the cold trap, it coolsuntil it condenses into liquid water, which will be used subsequentlyfor measuring the tritium ³H contained in this water.

One drawback of the device and method of this document is thatdiscretising the solid surface is only possible in fractions of the sizeof the connection head. However, the size of the connection head must besufficiently large to make it possible to aspirate a sufficient quantityof water in vapour form especially since the concrete does not containan enormous amount thereof. This technique also assumes that the tritium³H is labile whereas it may very well be strongly bonded to theconcrete.

Another drawback is the need to transport potentially radioactive liquidsamples.

Yet another drawback is the difficulty of robotising the placing of theconnection head, which has a specific form.

PRESENTATION

One objective of the invention is to overcome at least one drawback ofthe prior art described given above by way of examples.

To this aim, the invention provides a kit for detecting β⁻ radiations ona solid surface in a way that is not destructive to the solid surface,comprising at least two films sensitive to at least two distinct type ofradiation, including the β⁻ type.

One advantage of this kit is the simplicity of use thereof. In addition,the use of simple films allows robotised placing, which avoids a personentering the enclosure to be dismantled to carry out samplingoperations.

Another advantage is the possibility of having a detection in twodimensions of the solid surface.

Other optional and non-limitative features are:

-   -   the kit also comprises a protection for protecting the films        from the radiation surrounding the solid surface;    -   the protection is a sheet of polymer film;    -   the protection is a black fabric;    -   the protection is a cover made from a material that is not        penetrated by the radiation to which the films are sensitive;    -   the cover is made from lead;    -   a calibrated scale for quantifying the β⁻ radiations.

The invention also provides a method of using the kit described abovecomprising the following steps:

-   -   placing a first film on the solid surface;    -   placing a second film on the first film.

Other optional and non-limitative features of the method are:

-   -   it also comprises the step of covering the two films with the        protection;    -   it also comprises the step of developing the films by laser by        means of a suitable apparatus.

PRESENTATION OF THE DRAWINGS

Other objectives, features and advantages will emerge from reading thedetailed description that follows, with reference to the drawings givenby way of illustration and non-limitatively, among which:

FIG. 1 shows schematically a kit for the detection of β⁻ radiation inplace on a solid surface;

FIG. 2 a illustrates the steps of the method of using the kit of FIG. 1when a sampling scale is used;

FIG. 2 b illustrates the steps of the method of using the kit of FIG. 1when a separator is used;

FIG. 3 a is a reproduction of a first film developed by laser afterexposure to the radiations from the solid surface, the film being theone that is closer to the solid surface;

FIG. 3 b is a reproduction of a second film developed by laser afterexposure to the radiations of the solid surface, the film being the onefarther away from the solid surface;

FIG. 4 a illustrates the marking on a film due to a sampling scale forquantifying the β⁻ and therefore tritium ³H radiations obtained on afilm of the same material as the first film in FIG. 1;

FIG. 4 b is a reproduction of a developed film that has been placed ontop of the film carrying the marking of the sampling scale of FIG. 4 a.

DETAILED DESCRIPTION β⁻ Radiation Detection Kit

With reference to FIG. 1, a β⁻ radiation detection kit is describedbelow. This kit K makes it possible to detect, or even measure, β⁻radiation in a way that is not destructive to the solid surface S.

The kit K comprises at least two films 1, 2 sensitive to at least twodistinct types of radiation, including the β⁻ type.

For example, the two films 1, 2 may be identical and sensitive to β⁻, γ,X or even also α radiations.

In another example, the two films 1, 2 are different, one is sensitiveto β⁻ radiations and the other to γ radiations, or even also X but notto β⁻ radiations.

The number of films in the kit K varies according to the type ofradiations that it is wished to detect and/or measure.

The films 1, 2 are advantageously made from polymer covered with a layerof phosphorus. These films typically have a thickness of 1 mm. Thesefilms 1, 2, once exposed to the β⁻, γ, X or α radiations, can easily bereused by exposing them to an intense homogeneous white light butwithout UV.

The kit K may also comprise a protection 3 for protecting the films 1, 2from the other radiations surrounding the solid surface S and inparticular to the radiation of daylight, which could wipe the films. Theprotection 3 may be a thin sheet of polymer of approximately 450 μg/cm²,a black fabric or a cover made from a material that is not penetrated bythe radiations to which the films 1, 2 are sensitive. In the case wherethe protection 3 is a cover, this may be made from lead.

The β⁻ radiations are emitted by the nuclei of various atoms, forexample tritium ³H and carbon 14 ¹⁴C. In the case where the concern issolely with tritium ³H, it is necessary to detect only the β⁻ radiationsof tritium ³H. The β⁻ radiations emitted by carbon 14 (¹⁴C) are morepenetrating that the β⁻ radiations emitted by tritium ³H. However, theβ⁻ radiations from carbon 14 ¹⁴C may be separated from the β⁻ radiationsfrom tritium ³H using a separator 4 consisting for example of a sheet ofpaper between the solid surface S and the first film 1. In this caseonly carbon 14 ¹⁴C is seen on the development of the film. The tritium³H is then observed by the difference between two recordings, one withthe separator and the other without the separator.

The kit K may also comprise a sampling scale 5 for quantifying thetritium ³H. The sampling scale 5 makes it possible to quantify the β⁻radiations and therefore to quantify tritium ³H.

The sampling scale 5 is a series of diverse matrices (for examplepolymer, concrete, metals, etc.) comprising tritium ³H in differentconcentrations, for example from 1 kBq/g to around 10 kBq/g, thusforming a set of standards. The sampling scale 5 is to be placed on thesolid surface S and under the films 1, 2. The matrices are chosen so asto obtain a good contrast on the film 1 (the size of the matrix in theexample in FIG. 4 a corresponds to a cylinder with a diameter ofapproximately 1 cm and a thickness of approximately 1 mm).

The sampling scale impresses the films 1, 2 differently according to theexposure time. In the case of quantitative analysis, it is necessary forthe films 1, 2 to be exposed for the same period of time on the scaleand the materials to be analysed,

Method of Use of the Kit

With reference to FIGS. 2 a and 2 b, a description is given below of amethod of using the kit K described above.

First of all, a first film 1 is placed E2, E2′ on the solid surface Sdirectly in contact therewith. This first film 1 will be marked by theβ⁻, γ, X and a radiations and stop some of them.

Next, a second film 2 is placed E3, E3′ on the first film 1 directly incontact therewith. Some radiations have been stopped by the first film 1and therefore the second film 2 will be marked only by the radiationsthat have passed through the first film 1 and therefore that arepenetrating.

Other films can also be placed on each occasion by stacking on theprevious stack and in contact therewith. The number of films to be useddepends on the β⁻ radiations that it is wished to detect and/or measure.This is because not all β⁻ radiations penetrate in a material in thesame way and over the same distance.

β⁻ radiations are not very penetrating; in the majority of cases theyare stopped by the first film 1 and therefore mark only the latter.However, in some cases, β⁻ radiations may be more penetrating. It istherefore necessary to use at least one additional film. The mostpenetrating radiations such as γ or X mark all the films.

It is possible to verify whether the radiations detected on the secondfilm 2 or, if several films are used, the last on the top of the stack,are radiations other than β⁻ by using a suitable electronic detector. Ifthe detector indicates that the radiations that mark the second film 2or, in the case where several films have been used, the last film on thetop of the stack, are γ or X radiations for example, then there is noneed to add another film. Otherwise, if the detector indicates thatthere exists a point on the second film 2 or, where applicable, the lastfilm on the top of the stack, which is not marked by γ or X radiations,for example, then it will be necessary to add an additional film sincethe mark left is probably due to more penetrating β⁻ radiations.

In order to avoid as much as possible the impression and/or obliterationof the films by stray light coming from the environment around the solidsurface S, the two films 1, 2 or the stack of films can be covered E4,E4′ with a protection 3, for example a cover.

The films 1, 2 are left exposed to the radiations from the solid surfaceS for a given time, for example two weeks in the case of lowcontamination with tritium ³H. The time is determined experimentally inorder to optimise the signal to background ratio and obtain the bestcontrast on the film.

At the end of the exposure time, the films 1, 2 are collected andexposed by laser by means of suitable apparatus.

When the films 1, 2 are collected and optionally transported, it will benecessary to take care not to leave them exposed to ambient stray lightso as to prevent deletion of the data recorded by the films 1, 2.

Where a separator 4 is used, FIG. 2 b, it is placed E1′ before the firstfilm 1 directly in contact with the solid surface S. In this case, thefirst film 1 is then in contact with the separator 4.

If a sampling scale 5 in the form of a series of matrices is used (FIG.2 a) comprising tritium ³H in different concentrations, it must beplaced E1 under the first film 1 or, where applicable, under theseparator 4.

One advantage of this method is the simplicity of the successiveoperations to be put in place and which are summarised in thesuperimposition of films 1, 2 and optionally separator 4, protection 3and sampling scale 5. Consequently the robotisation of these operationsis simplified and the robot to be developed is not of great complexity.

Results Obtained with the Kit

FIGS. 3 a and 3 b reproduce the markings of the first (FIG. 3 a) andsecond (FIG. 3 b) films 1 and 2 left by the β⁻ and γ radiations emittedby a solid surface that has been in contact with tritium ³H.

In FIG. 3 a, 8 marked regions can be seen, they are referenced B1 to B3and G1 to G5 on the first film 1.

In FIG. 3 b, 5 marked regions can be seen, they are referenced Γ1 to Γ5on the second film 2.

By comparing the two films 1 and 2, it can be noted that the regions G1to G5 of the first film 1 correspond to the regions Γ1 to Γ5 of thesecond film 2. These five regions have therefore been marked by γradiations more penetrating than the β⁻ radiations, which for their parthave only marked the regions B1 to B3 of the first film 1. It is theabsence of marking of the corresponding regions of the second film 2that indicates the β⁻ type of these radiations.

The areas of the solid surface corresponding to the areas B1 to B3 havetherefore been contaminated by tritium ³H.

This demonstrates that the detection of the β⁻ radiations (andindirectly of the atom nuclei responsible for these radiations) issimplified. In addition, this detection gives information on the tritium³H contamination in two dimensions.

In order to be able to measure the β⁻ radiations (and consequentlytritium ³H) quantitatively, a sampling scale 5 is used as illustrated inFIG. 4 a. It will be noted that the marking left by the sampling scale 5is a succession of circles more or less intensely marked (dark). Thecloser the circle is to the colour of the rest of the film, the lowerthe concentration of tritium ³H. By comparing the result obtained forthe first film 1 with this scale 5, it is possible to quantify thetritium ³H.

FIG. 4 b shows a film placed in contact with the film of FIG. 4 a. Thisfilm is not marked. Which shows clearly that the β⁻ radiations can becharacterised by the stacking of two films and are present at a pointprovided both that the first film 1 is marked and that the second film 2is not marked.

1. A kit for the detection of β⁻ radiations coming from a solid surfacein a way that is not destructive to the solid surface, comprising atleast two films sensitive to at least two distinct types of radiationincluding the β⁻ type.
 2. The kit of claim 1, further comprising aprotection for protecting the films from the radiations surrounding thesolid surface.
 3. The kit of claim 2, wherein the protection is a sheetof polymer film.
 4. The kit of claim 2, wherein the protection is ablack fabric.
 5. The kit of claim 2, wherein the protection is a covermade from a material that cannot be penetrated by the radiations towhich the films are sensitive.
 6. The kit of claim 5, wherein the coveris made from lead.
 7. The kit of claim 1, further comprising a scalecalibrated for quantifying β⁻ radiations.
 8. A method comprising:placing on the solid surface a first film or a superimposition of anumber of films greater than two of the kit of claim 1; placing a secondfilm of the kit on the first film or the superimposition of films;developing the films; and comparing the first film or the film of thesuperimposition of films positioned the closest to the solid surfacewith the second film after their development, wherein the number offilms is determined so that no β⁻ radiation reaches the second film. 9.The method of claim 8, also comprising: covering the two films with theprotection.
 10. The method of claim 8 wherein the step of developing thefilms is carried out with a laser using suitable apparatus.